The sensors with a unified current output 4-20, 0-50 or 0-20 mA can have the most distribution in the field of automation of production can have different connection circuits to secondary instruments. Modern sensors with low power consumption and current yield 4-20 mA are most often connected by two-wire diagram. That is, only one cable with two veins is connected to such a sensor, for which this sensor is powered, and the transmission is carried out according to the same two veins.

As a rule, the sensors with a 4-20 mA output and a two-wire connection scheme have a passive output and an external power supply is needed to work. This power source can be built directly into the secondary instrument (in its input) and when connecting the sensor to such a device in the signal circuit, the current immediately appears. About devices that have a power supply built-in input for a sensor, they say that these are appliances with an active input.

Most modern secondary instruments and controllers have built-in power supplies to work with passive exit sensors.

If the secondary instrument has a passive input - in fact, simply resistor with which the measuring scheme of the device "reads" the voltage drop proportional to the current flowing in the circuit is necessary for the operation of the sensor. The external power supply in this case is turned on sequentially with the sensor and the secondary instrument in the tip of the current loop.

Secondary devices are usually designed and manufactured with such a calculation so that it can be connected to them as two-wire sensors 4-20 mA and sensors 0-5, 0-20 or 4-20 mA, connected along a three-wire diagram. To connect a two-wire sensor to the input of the secondary instrument with three input terminals (+ u, input and general) use the terminals "+ U" and "input", the "Common" terminal remains free.

Since the sensors, as mentioned above, may not only have a yield of 4-20 mA, and, for example, 0-5 or 0-20 mA or cannot be connected according to a two-wire diagram due to the large-scale energy consumption (more than 3 mA) , then use a three-wire connection scheme. In this case, the power supply chain of the sensor and the output circuit is separated. Sensors having a three-wire connection scheme usually have an active output. That is, if you submit the supply voltage to the sensor with an active output and between its output terminals "output" and "general" to connect the load resistance, then the output circuit will run the current, proportional to the value of the measured parameter.

Secondary instruments usually have a sufficiently low-power built-in power supply for the sensors. The maximum output current of the built-in power supplies is usually located within 22-50 mA, which is not always enough to power the sensors having a large power consumption: electromagnetic flow meters, infrared gas analyzers, etc. In this case, to power the three-wire sensor, you have to use an external, more powerful power supply, providing the required power. The power supply built into the secondary instrument is not used.

A similar scheme for turning on three-wire sensors is usually used and in the case when the voltage of the power supply unit does not match the power supply, which is allowed to be supplied to this sensor. For example, a built-in power supply has a 24V output voltage, and the sensor is allowed to feed the voltage from 10 to 16.

Some secondary devices can have several input channels and a powerful powerful power supply for external sensors. It must be remembered that the total power consumption of all sensors connected to such multichannel sensors must be less than the power of the built-in power source intended for their power. In addition, studying the technical characteristics of the device must clearly distinguish between the assignment of the blocks built into it (sources). One built-in source is used to power directly the secondary instrument itself - for the operation of the display and indicators, output relays, electronic diagram of the device, etc. This power source may have quite large power. The second built-in source is used to use exclusively input circuits - sensors connected to inputs.

Before connecting the sensor to the secondary device, you should carefully examine the operation manual for this equipment, to determine the types of inputs and outputs (active / passive), check the compliance with the power sensor consumed by the power supply sensor and power (built-in or external) and only after that connect. Real designations of the input and output terminals of sensors and instruments may differ from those that are presented above. So the terminals "Vx (+)" and "Vx (-)" may have the designation + j and -j, + 4-20 and -4-20, + in and -in, etc. Terminal "+ U Pete" can be marked as + v, supply, + 24v, etc., "Exit" terminal - Out, Sign, Jout, 4-20 mA, etc., Terminal "General" - GND , -24v, 0v, etc., but it makes no sense.

Current output sensors having a four-wire connection scheme have a similar connection scheme as two-wire sensors with the only difference that the power of the fetal sensors is carried out on a separate pair of wires. In addition, four-wire sensors may have as it should be considered when selecting a connection scheme.

Discrete sensors

Such an algorithm allows you to avoid a blow when the mold is closed, otherwise it can simply be chopped into small pieces. The same change of speed occurs when the mold is opened. Here two contact sensors can not do.

Application of analog sensors

Figure 2. Whitstone Bridge

Connecting analog sensors

Analog Sensor Output Signals

But the only sensor is usually nothing. One of the most popular dimensions are measuring temperature and pressure. The number of such points in modern industries can reach several tens of thousands. Accordingly, the number of sensors is also great. Therefore, several analog sensors are most often connected to one controller. Of course, not several thousand, well, well, if a dozen is different. Such a connection is shown in Figure 7.

Figure 7. Connecting multiple analog sensors to the controller

In this figure, it is shown how from the current signal it turns out a voltage suitable for conversion into a digital code. If there are several such signals, they are not all processed at once, but are separated by time, multiplexed, otherwise it would have to put a separate ADC to each channel.

For this purpose, the controller has channel switching scheme. The functional circuit of the switch is shown in Figure 8.

Figure 8. Channel switch of analog sensors (picture clickable)

The current loop signals converted to the voltage on the measuring resistor (UR1 ... URN) come to input an analog switch. Control signals alternately skip the output one of the Signals of UR1 ... URN, which are amplified by the amplifier, and alternately go to the ADC entry. The voltage transformed into the digital code enters the controller.

The scheme, of course, is very simplified, but the principle of multiplexing in it is quite possible. Approximately the input module of the analog signals of MSTC controllers (microprocessor system of technical means) issued by the Smolensky PC "Prolog" was built.

The release of such controllers has long been discontinued, although in some places, far from the best, these controllers still serve so far. The controllers of new models come to replace these museum exhibits, mainly imported (Chinese) production.

If the controller is mounted in a metal cabinet, then the shielding braids are recommended to connect to the grounding point of the cabinet. The length of the connecting lines can reach two more than kilometers, which is calculated by the appropriate formulas. We will not count here, but believe me that it is.

New sensors, new controllers

With the arrival of new controllers, new analog sensors operating via HART (Highway Addressable Remote Transducer) appeared, which is translated as a "measuring transducer addressing remotely through the highway".

The output signal of the sensor (field device) is an analog current signal of the range of 4 ... 20mA, which is superimposed by frequency modulated (FSK - Frequency SHIFT KEYING) The digital communication signal.

It is known that the average value of the sinusoidal signal is zero, therefore, on the output current of the sensor 4 ... 20 mA, the transmission of digital information does not have the effect. This mode is used when setting up sensors.

Communication under the HART protocol is carried out in two ways. In the first case, the standard, only two devices can exchange information on the two-wire line, while the output analog signal 4 ... 20mA depends on the measured value. This mode is used when setting up field devices (sensors).

In the second case, up to 15 sensors can be connected to a two-wire line, the number of which is determined by the communication line parameters and power supply power. This is multipoint communication mode. In this mode, each sensor has its own address in the range of 1 ... 15, along which the control device is referred to.

The sensor with the address 0 from the communication line is disabled. Data exchange between the sensor and the control device in multipoint mode is carried out only by the frequency signal. The current sensor signal is fixed at the required level and does not change.

Under the data in the case of multipoint communication implies not only the actual measurement results of the controlled parameter, but also a whole set of all sorts of service information.

First of all, it is the sensor addresses, control commands, settings. And all this information is transmitted on two-wire lines of communication. Is it possible to get rid of them? True, it is necessary to do it carefully, only in cases where the wireless connection cannot affect the safety of the controlled process.

These technologies have replaced the old analog current loop. But she does not surrender its position, is widely used everywhere, where possible.

In the process of automation of technological processes, mechanisms and aggregates have to be faced with measurements of various physical quantities. This may be the temperature, pressure and consumption of fluid or gas, the speed of rotation, the light strength information about the position of the parts of the mechanisms and much more. This information is obtained using sensors. Here, first about the position of parts of the mechanisms.

Discrete sensors

The easiest sensor is the usual mechanical contact: the door opened - the contact turned around, closed - closed. Such a simple sensor, as well as the above work algorithm, is often used in security alarms. For a mechanism with a progressive movement, which has two positions, for example, the tap valve will already need two contacts: one contact closed - the valve closed, closed another - closed.

A more complex algorithm of the progressive movement has a mechanism for closing the mold thermoplastic machine gun. Initially, the mold is open, this is the initial position. In this position, finished products are extracted from the mold. Next, the worker closes the protective fence and the mold begins to close, the new working cycle begins.

The distance between the molds is large enough. Therefore, at first, the mold is moving quickly, and at some distance to the closure of the half, the terminal is triggered, the speed of movement is significantly reduced and the mold is smoothly closed.

Thus, contact-based sensors are discrete or binary, have two positions, closed - open or 1 and 0. In other words, it can be said that the event occurred or not. In the example above, the contacts are "captured" several points: the start of the movement, the speed of reduced speed, the end of the movement.

In geometry, the point does not have any sizes, just a point and that's it. It can either be (on a sheet of paper, in the trajectory of movement, as in our case) or simply no. Therefore, discrete sensors are used to detect points. There may be a comparison with the point here is not very appropriate, because for practical purposes the accuracy of the accuracy of the discrete sensor is used, and this accuracy is much more geometric point.

But the mechanical contact itself is unreliable. Therefore, wherever possible, mechanical contacts are replaced by contactless sensors. The easiest option is the German: the magnet approached, the contact closed. The accuracy of the opening of Herrkeon leaves much to be desired, apply such sensors just to determine the position of the doors.

A more complex and accurate option should consider various non-contact sensors. If the metal flag entered the slot, the sensor worked. As an example of such sensors, you can give the BVK sensors (contactless switch finite) different series. The accuracy of the response (differential stroke) of such sensors is 3 millimeters.

BVK series sensor

Figure 1. BVK series sensor

Power supply voltage BVK 24V, load current 200mA, which is enough to connect an intermediate relay for further matching with the control circuit. This is how the BVK sensors are used in various equipment.

In addition to the BVC sensors, sensors are also used sensors of BTP, CFCs, PIPs, QWs, Foods. Each series has several types of sensors indicated by numbers, for example, BTP-101, BTP-102, BTP-103, BTP-211.

All mentioned sensors are non-contact discrete, their main purpose determination of the position of parts of the mechanisms and aggregates. Naturally, these sensors are much larger, not to write about all in one article. Even more common and still find widespread use of various contact sensors.

Application of analog sensors

In addition to discrete sensors in automation systems, analog sensors are widely used. Their appointment is to obtain information about various physical quantities, and not, just so in general, but in real time. More precisely, the transformation of the physical quantity (pressure, temperature, illumination, consumption, voltage, current) into an electrical signal is suitable for transmission over communication lines to the controller and its further processing.

Analog sensors are located, as a rule, rather far from the controller, which often is often called field devices. This term is often used in technical literature.

Analog sensor, as a rule, consists of several parts. The most important part is a sensitive element - sensor. Its assignment to translate the measured value into an electrical signal. But the signal received from the sensor is usually small. To obtain a signal suitable for amplification, the sensor is most often included in the bridge circuit - Whitstone bridge.

Wheatstone bridge

Figure 2. Whitstone Bridge

The initial purpose of the bridge circuit is precise measurement of resistance. A DC source is connected to the AD bridge diagonal. A sensitive galvanometer is connected to another diagonal with a midwater, with zero in the middle of the scale. To measure the resistance of the resistor Rx, the rotation of the trim resistor R2 should achieve the axle equilibrium, set the galvanometer arrow to zero.

Deviation of the appliance arrow in one direction or another allows you to determine the direction of rotation of the resistor R2. The value of the measured resistance is determined by the scale combined with the R2 resistor handle. The equilibrium of the bridge is equality of relations R1 / R2 and RX / R3. In this case, between points BC, the zero difference of potentials is obtained, and the current via the galvanometer V does not proceed.

Resistance to resistors R1 and R3 is selected very precisely, their scatter must be minimal. Only in this case, even a small loss of the bridge causes a rather noticeable change in the voltage of the diagonal BC. It is this property of the bridge that is used to connect sensitive elements (sensors) of various analog sensors. Well, then everything is simple, the case of technology.

To use the signal received from the sensor, it requires its further processing, - gain and conversion to the output signal suitable for transmitting and processing the control circuit controller. Most often, the output signal of the analog sensors is the current (analog current loop), less frequently voltage.

Why exactly the current? The fact is that the output stages of analog sensors are built on the basis of current sources. This allows you to get rid of the effect on the output signal of the connecting lines resistance, use long length connecting lines.

Further transformation is simple enough. The current signal is converted into voltage, for which it is sufficient to skip through the resistor of the known resistance. The voltage drop on the measuring resistor is obtained by the law U \u003d I * R.

For example, for a current of 10 mA on a resistor resistance 100, a voltage is 10 * 100 \u003d 1000mV, as well as a whole 1 volt! In this case, the output current of the sensor does not depend on the resistance of the connecting wires. In reason, of course, the limits.

Connecting analog sensors

The voltage obtained on the measuring resistor is easy to convert to a digital view suitable for input to the controller. The transformation is performed using analog-digital ADC converters.

Digital data into the controller is transmitted by serial or parallel code. It all depends on the specific inclusion scheme. The simplified connection diagram of an analog sensor is shown in Figure 3.

Connect analog sensor

Figure 3. Connect analog sensor (to enlarge click on the picture)

Executive mechanisms are connected to the controller, or the controller itself connects to a computer that is included in the automation system.

Naturally, analog sensors have a complete design, one of the elements of which is a housing with connecting elements. As an example, Figure 4 shows the appearance of the excess pressure sensor of the probe-10 type.

Excessive pressure sensor probe 10

Figure 4. Excessive pressure sensor Pronductor-10

At the bottom of the sensor, you can see the connecting thread to connect to the pipeline, and on the right under the black lid there is a connector for connecting a link with the controller.

Sealing the threaded connection is made using an annealed copper washable (included in the package of the sensor), and not by winding from Fium-tape or flax. This is done in order that when installing the sensor, do not deform the sensor item located inside.

Analog Sensor Output Signals

According to standards, there are three range of current signals: 0 ... 5mA, 0 ... 20mA and 4 ... 20mA. What is their difference, and what are the features?

Most often, the dependence of the output current is directly proportional to the measured value, for example, the higher the pressure in the pipe, the greater the current at the output of the sensor. Although the inverse inclusion is sometimes used: the larger value of the output current corresponds to the minimum value of the measured value at the output of the sensor. It all depends on the type of controller used. Some sensors even switch from direct to the inverse signal.

The output signal of the range 0 ... 5ma is very small, and therefore is subject to interference. If the signal of such a sensor varies with the constant value of the parameter measured, that is, the recommendations are parallel to the sensor output to set the capacitor with a capacity of 0.1 ... 1MKF. A current signal in the range of 0 ... 20mA is more stable.

But both of these slope of the fact that zero at the beginning of the scale does not allow to unambiguously determine what happened. Or the measured signal actually accepted the zero level, which in principle maybe or simply broke the link line? Therefore, the use of these ranges is trying, if possible, refuse.

The signal of analog sensors with output current is considered more reliable in the range of 4 ... 20mA. His noiselessness is high enough, and the lower limit, even if the measured signal has a zero level, will be 4th, which suggests that the communication line is not converted.

Another good feature of the range of 4 ... 20mA is that the sensors can be connected in total on two wires, since it is such a current that the sensor itself is powered. This is its consumption current and simultaneously measuring signal.

The power supply of the range of 4 ... 20mAs is turned on, as shown in Figure 5. At the same time, the probe-10 sensors, like many others, have a wide range of supply voltage 10 ... 38B, although stabilized sources with a voltage 24V are most often used.

Connect analog sensor with an external power source

Figure 5. Connect analog sensor with an external power source

This scheme contains the following elements and notation. RS - resistor measuring shunt, RL1 and RL2 - resistance of communication lines. To increase the measurement accuracy, a precision measuring resistor should be used as a RS. Passage of current from the power source is shown by arrows.

It is easy to see that the output current of the power supply runs from the terminal + 24V, through the RL1 line reaches the sensor terminal + AO2, passes through the sensor and through the output contact of the sensor - AO2, the connecting line RL2, the RS resistor returns to the power supply terminal -24V. Everything, the chain closed, current flows.

In case the controller contains 24V power supply, the sensor or the measuring transducer is connected according to the diagram shown in Figure 6.

Connect analog sensor to the controller with an internal power source

Figure 6. Connect an analog sensor to the controller with an internal power source

This scheme shows another element - Ballast resistor RB. Its assignment of the protection of the measuring resistor when closing a communication line or analog sensor fault. Installing the RB resistor is optional, although desirable.

In addition to different sensors, the current output also has measuring transducers, which in automation systems are used quite often.

The measuring transducer is a device for converting voltage levels, for example, 220V or current to several tens or hundreds of amps into a current signal 4 ... 20mA. There is simply a transformation of the level of the electrical signal, and not the representation of some physical size (speed, consumption, pressure) in electrical form.

But the only sensor is usually nothing. One of the most popular dimensions are measuring temperature and pressure. The number of such points in modern industries can reach several tons

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Here I also ruled such an important practical question as connecting inductive sensors with a transistor outlet, which in modern industrial equipment - everywhere. In addition, real instructions for sensors and references to examples are presented.

The principle of activation (operation) of the sensors can be any - inductive (approximations), optical (photoelectric), etc.

In the first part, possible options for sensor outputs were described. On the connection of sensors with contacts (relay output) problems should not occur. And on transistor and with connection to the controller, not so simple.

PNP and NPN Sensor Connection Schemes

The difference between PNP and NPN sensors in the fact that they commute different poles of the power supply. PNP (from the word "POSITIVE") commutes a positive output of the power supply, NPN is negative.

Below for example, there is a scheme for connecting sensors with a transistor output. Load - usually this is the input of the controller.

Sensor. Load (LOAD) is constantly connected to "minus" (0v), the supply of discrete "1" (+ v) is switched by the transistor. But or NZ Sensor - depends on the control scheme (Main Circuit)

Sensor. Load (Load) is constantly connected to the "plus" (+ v). Here is the active level (discrete "1") at the output of the sensor - low (0v), and the load is powered through the opened transistor.

I urge everyone not to confuse, the work of these schemes will be described in detail further.

The schemes below shows in principle the same. The emphasis is on the differences in the PNP and NPN output schemes.

Schemes for connecting NPN and PNP sensor outputs

On the left drawing - the sensor with the output transistor NPN.. A common wire is switched, which in this case is a negative power source wire.

Right - case with a transistor PNP. at the exit. This case is the most frequent, since in modern electronics adopted negative power source wire to make shared, and the inputs of controllers and other registering devices are activated by the positive potential.

How to check inductive sensor?

To do this, apply nutrition on it, that is, to connect it into the scheme. Then - activate it (initiate) it. When activated, the indicator will light up. But the indication does not guarantee the proper operation of the inductive sensor. You need to connect the load, and measure the voltage on it to be 100% confident.

Replacing sensors

As I already wrote, there are fundamentally 4 types of sensors with a transistor output, which are divided by an internal device and an inclusion scheme:

• PNP No.
• PNP NC.
• NPN No.
• NPN NC.

All these types of sensors can be replaced by each other, i.e. They are interchangeable.

This is implemented in such ways:

• Alteration of the initiation device - the design mechanically changes.
• Changing the existing sensor inclusion scheme.
• Switching a sensor output type (if there are switches on the sensor housing).
• Reprogramming of the program is a change in the active level of this entry, a change in the program algorithm.

Below is an example of how to replace the PNP sensor on the NPN by changing the connection scheme:

PNP-NPN interchangeability schemes. On the left - the original scheme, on the right - converted.

To understand the work of these schemes will help the awareness of the fact that the transistor is a key element that can be represented by conventional relay contacts (examples below, in the notation).

So, the scheme on the left. Suppose that the type of sensor is but. Then (regardless of the type of transistor at the output), when the sensor is not active, its output "Contacts" is open, and the current does not proceed through them. When the sensor is active, contacts are closed, with all the ensuing consequences. More precisely, with flowing current through these contacts)). The flowing current creates a voltage drop on the load.

The internal load is shown by a dotted line. This resistor exists, but its presence does not guarantee a stable operation of the sensor, the sensor must be connected to the input of the controller or other load. Resistance to this entry and is the main load.

If there is no internal load in the sensor, and the collector is "hanging in the air", then this is called the "open collector" scheme. This scheme only works with a connected load.

So, in a diagram with PNP, the output when activating the voltage (+ v) through the open transistor enters the input of the controller, and it is activated. How to achieve the same with the NPN exit?

There are situations where the desired sensor is not at hand, and the machine should work "straight right now."

We look at changes in the scheme on the right. First of all, the operation mode of the output transistor of the sensor is provided. For this, an additional resistor is added to the scheme, its resistance is usually about 5.1 - 10 com. Now that the sensor is not active, through an additional resistor voltage (+ v) enters the controller input, and the input of the controller is activated. When the sensor is active - at the input of the controller is discrete "0", since the input of the controller is shunted by an open NPN transistor, and almost all current of the additional resistor passes through this transistor.

In this case, the sensor operation is reaffated. But the sensor works in mode, and the controller receives information. In most cases, this is sufficient. For example, in the calculation mode of pulses - a tachometer, or the number of blanks.

Yes, not exactly what we wanted, and the schemes of the interchangeability of NPN and PNP sensors are not always acceptable.

How to achieve full functional? Method 1 - mechanically move or redo the metal plate (activator). Either the light gap, if we are talking about the optical sensor. Method 2 - reprogram the input of the controller so that the discrete "0" was the active state of the controller, and "1" - passive. If there is a laptop at hand, then the second method and faster, and easier.

Conditional designation of the approximation sensor

On the concept schemes, inductive sensors (approximation sensors) are designated differently. But the main thing - there is a square turned on 45 ° and two vertical lines in it. As in the schemes shown below.

But NZ sensors. Circuits.

On the top scheme - normally open (but) contact (the PNP transistor is conventionally designated). The second scheme is normally closed, and the third scheme is both contacts in one case.

Color marking of sensor outputs

There is a standard sensor marking system. All manufacturers are currently holding it.

However, it is not necessary to make sure to make sure that the connection is correct, contacting the manual (instructions) on the connection. In addition, as a rule, the colors of the wires are indicated on the sensor itself, if it allows its size.

Here is this marking.

• Blue (Blue) - minus power
• Brown (Brown) - plus
• Black (Black) - Exit
• White (White) - second output, or control, We need to look at the instruction.

Inductive sensor designation system

The sensor type is indicated by a digital-letter code, which encrypts the main sensor parameters. Below is the labeling system of popular autonics sensors.

Download instructions and manuals for some types of inductive sensors: I meet in my work.

Thank you all for your attention, waiting for questions on connecting sensors in the comments!